mm/nommu: factor out check for NOMMU shared mappings into is_nommu_shared_mapping()

Patch series "mm/nommu: don't use VM_MAYSHARE for MAP_PRIVATE mappings".

Trying to reduce the confusion around VM_SHARED and VM_MAYSHARE first
requires !CONFIG_MMU to stop using VM_MAYSHARE for MAP_PRIVATE mappings.
CONFIG_MMU only sets VM_MAYSHARE for MAP_SHARED mappings.

This paves the way for further VM_MAYSHARE and VM_SHARED cleanups: for
example, renaming VM_MAYSHARED to VM_MAP_SHARED to make it cleaner what is
actually means.

Let's first get the weird case out of the way and not use VM_MAYSHARE in
MAP_PRIVATE mappings, using a new VM_MAYOVERLAY flag instead.


This patch (of 3):

We want to stop using VM_MAYSHARE in private mappings to pave the way for
clarifying the semantics of VM_MAYSHARE vs. VM_SHARED and reduce the
confusion. While CONFIG_MMU uses VM_MAYSHARE to represent MAP_SHARED,
!CONFIG_MMU also sets VM_MAYSHARE for selected R/O private file mappings
that are an effective overlay of a file mapping.

Let's factor out all relevant VM_MAYSHARE checks in !CONFIG_MMU code into
is_nommu_shared_mapping() first.

Note that whenever VM_SHARED is set, VM_MAYSHARE must be set as well
(unless there is a serious BUG). So there is not need to test for
VM_SHARED manually.

No functional change intended.

Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: David Hildenbrand <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Jens Axboe <[e-mail]>
Cc: Nicolas Pitre <[e-mail]>
Cc: Pavel Begunkov <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

drivers/char/mem.c

···
 /* can't do an in-place private mapping if there's no MMU */
 static inline int private_mapping_ok(struct vm_area_struct *vma)
 {
-	return vma->vm_flags & VM_MAYSHARE;
+	return is_nommu_shared_mapping(vma->vm_flags);
 }
 #else
 

fs/cramfs/inode.c

···
 
 static int cramfs_physmem_mmap(struct file *file, struct vm_area_struct *vma)
 {
-	return vma->vm_flags & (VM_SHARED | VM_MAYSHARE) ? 0 : -ENOSYS;
+	return is_nommu_shared_mapping(vma->vm_flags) ? 0 : -ENOSYS;
 }
 
 static unsigned long cramfs_physmem_get_unmapped_area(struct file *file,

fs/proc/task_nommu.c

···
 		}
 
 		if (atomic_read(&mm->mm_count) > 1 ||
-		    vma->vm_flags & VM_MAYSHARE) {
+		    is_nommu_shared_mapping(vma->vm_flags)) {
 			sbytes += size;
 		} else {
 			bytes += size;

fs/ramfs/file-nommu.c

···
  */
 static int ramfs_nommu_mmap(struct file *file, struct vm_area_struct *vma)
 {
-	if (!(vma->vm_flags & (VM_SHARED | VM_MAYSHARE)))
+	if (!is_nommu_shared_mapping(vma->vm_flags))
 		return -ENOSYS;
 
 	file_accessed(file);

fs/romfs/mmap-nommu.c

···
  */
 static int romfs_mmap(struct file *file, struct vm_area_struct *vma)
 {
-	return vma->vm_flags & (VM_SHARED | VM_MAYSHARE) ? 0 : -ENOSYS;
+	return is_nommu_shared_mapping(vma->vm_flags) ? 0 : -ENOSYS;
 }
 
 static unsigned romfs_mmap_capabilities(struct file *file)

include/linux/mm.h

···
 	return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;
 }
 
+#ifndef CONFIG_MMU
+static inline bool is_nommu_shared_mapping(vm_flags_t flags)
+{
+	/*
+	 * NOMMU shared mappings are ordinary MAP_SHARED mappings and selected
+	 * R/O MAP_PRIVATE file mappings that are an effective R/O overlay of
+	 * a file mapping. R/O MAP_PRIVATE mappings might still modify
+	 * underlying memory if ptrace is active, so this is only possible if
+	 * ptrace does not apply. Note that there is no mprotect() to upgrade
+	 * write permissions later.
+	 */
+	return flags & VM_MAYSHARE;
+}
+#endif
+
 #if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
 #define SECTION_IN_PAGE_FLAGS
 #endif

io_uring/io_uring.c

···
 
 static int io_uring_mmap(struct file *file, struct vm_area_struct *vma)
 {
-	return vma->vm_flags & (VM_SHARED | VM_MAYSHARE) ? 0 : -EINVAL;
+	return is_nommu_shared_mapping(vma->vm_flags) ? 0 : -EINVAL;
 }
 
 static unsigned int io_uring_nommu_mmap_capabilities(struct file *file)

mm/nommu.c

···
 	 */
 	if (capabilities & NOMMU_MAP_DIRECT) {
 		ret = call_mmap(vma->vm_file, vma);
+		/* shouldn't return success if we're not sharing */
+		if (WARN_ON_ONCE(!is_nommu_shared_mapping(vma->vm_flags)))
+			ret = -ENOSYS;
 		if (ret == 0) {
-			/* shouldn't return success if we're not sharing */
-			BUG_ON(!(vma->vm_flags & VM_MAYSHARE));
 			vma->vm_region->vm_top = vma->vm_region->vm_end;
 			return 0;
 		}
···
 	 *   these cases, sharing is handled in the driver or filesystem rather
 	 *   than here
 	 */
-	if (vm_flags & VM_MAYSHARE) {
+	if (is_nommu_shared_mapping(vm_flags)) {
 		struct vm_region *pregion;
 		unsigned long pglen, rpglen, pgend, rpgend, start;
 
···
 		for (rb = rb_first(&nommu_region_tree); rb; rb = rb_next(rb)) {
 			pregion = rb_entry(rb, struct vm_region, vm_rb);
 
-			if (!(pregion->vm_flags & VM_MAYSHARE))
+			if (!is_nommu_shared_mapping(pregion->vm_flags))
 				continue;
 
 			/* search for overlapping mappings on the same file */
···
 	if (vma->vm_end != vma->vm_start + old_len)
 		return (unsigned long) -EFAULT;
 
-	if (vma->vm_flags & VM_MAYSHARE)
+	if (is_nommu_shared_mapping(vma->vm_flags))
 		return (unsigned long) -EPERM;
 
 	if (new_len > vma->vm_region->vm_end - vma->vm_region->vm_start)